Television system



LE ROY MOFFETT ,363, v

I TELEVISION SYSTEM V Filed March' 51, 1942 2 Sheets-Sheet 1 Mn Y R E 4 r w 0 M Halo/Hf H E 8 00 T J O e 3 6 4 6 6 LB 2 f a 2 a 5 M O 2 7mm um v .v Y m 2 a: up) ii: pun-b T M L. 1. 6 0 a VW a 4 w M @/8 v a ,8 g 4 a f. 4. m OP. P W W W I, H, I|u= I w NW, 2, 1944-. LE ROY MOFFETT TELEVISION SYSTEM Filed March 51,- 1942 2 Sheets-Sheet 2 lu l: Iii- ATTORNEY Patented Nov; 28, i944 TELEVISION srs'raM Le, Roy Moiiett, Baldwin, N. Y., aaaignor to Radio Corporation ware - Application March :1. 1992", Serial N6. 4mm

of America, a corporation of Dela- 8 Claims. [(01. ran-v.2)

' I vThis invention relates to an improvement in teievisiontransmitting systems and more particularly to an improvement in arrangements for the reinsertion of the direct current picture signal component at the television transmitter.

In television transmitter equipment a transmitting tube is provided that functions to convert an optical light image into a series of picture signals. These picture signals constitute a very wide range of frequencies, the range being from substantially a direct current variation to frequencies of the order of six megacycles or more per second. The upper limit of the produced picture signal frequencies is a function of the subject matter scanned and the rate at which the scanning is performed, The relatively slow or low frequency potential variations are representative of the background or average brilliance of the picture rather than the detail thereof since, for example, in a picture that is predominantly black a certain component of low frequency may be produced whereas in a picture that is predominantly white a different low frequency and direct current component will be produced. Since the detail in the picture is represented by the relatively high frequencies, it is necessary that these high frequencies be transmitted to a receiver in order that the corresponding detail may be reproduced at the receiver. Likewise, inasmuch as the background or average brilliance of the picture is represented by the relatively low'frequency potential variations and the direct current component, it is necessary that this portion of the picture signal series be transmitted in order to produce a proper image at the television receiver.

Inasmuch as. the picture signals that are derived directiy from the television transmitting tube are of relatively low intensity, it is necessary that the signals be subjected to several stages of amplification and, inasmuch as the use'of alternating current coupled stages of amplification are considerably more commercially feasible than direct coupled amplifiers, the low frequency component and the direct current component-of the picture. signal series are blocked in the'picture signal amplifier so that only frequencies above a certain predetermined frequency are passed through transmit the entire picture signal series. It has been found, however, that conventional methods of reinserting the direct current component do not insert that component at the proper value due to inherent losses in the usual direct current insertion circuit.

In order that the television receiving apparatus may be maintained in accurate synchronous operation with the television transmitter, a series of synchronizing impulses aretransmitted, the syn chronizing impulses occurring at both the horizontal and the vertical deflection rates. These 1 synchronizing impulses are of relatively short sponding'to black may vary: 2.5% from 75%.

' and amplified by the picture signal amplifier.

order that the low frequency component and direct current component of the picture signal series .may be transmitted it is customary to reinsert the direct current component in the picture signal'series just prior to the conventional radio frequency modulator tube. By'direct coupling from the modulator tube it is then possible tot} duration and are interspersed with the picture signals so that they occur in synchronism with the deflections of the scanning cathode ray beam and are phased with respect to the deflection such that they are positioned in the picture signal series at instances corresponding to the return deflection of the scanning cathode ray beam in the television transmitter. Since no useful picture signals are generated at these intervals (or at periods corresponding ,to the scanning beam blanking interval), the insertion of the horizontal and vertical synchronizing impulses at such time does not interfere with the transmission of the picture signals and, furthermore, the transmisof maximum power output. Signals representative of'white in the picture are then transmitted at values representative'of lesser degrees ofmodulation. Inasmuch as the synchronizing signals may always be transmitted at the same intensity.

thereis generally no variation in the differential percentage of modulation in so. far asthe synchronizing signals are. concerned as compared with black since the signals are transmitted at substantially maximum carrier they are generally referred to as extending ina blacker than black direction.

For the-transmission of picture signals and in order to fix a predetermined reference point with respect to which the alternating current components of the picture signals are transmitted, the Signal level of black in the picture (which corresponds to the signalstrength oi the pedestal or blanking signal upon which the ynchronizing signals are positioned) is generally taken as a reference point. Then the alternating current axis of all of thealtemating current components may be related to black in the picture. If, therefore, a picture is predominantly black the alternating current axis of the picture .signals per se will be moved to a point more nearly adjacent the levelv represented by black than would be the alternating current axis of a picture that is predominantly white. In the absence of a direct current insertion circuit the alternating current component of the picture signals would not be would appear "washed out" or lighter than should be the case. The insertion of the direct current component therefore shifts the alternating current axis of the picture signals with respect to black in the picture in order to thereby transmit an indication of the proper background level so that appropriate circuits in the receiver may respond to that transmission to vary the bias-between cathode and control electrode in the television reproducing tube to accordingly alter the average brilliance of the reproduced television image.

In presently used direct current insertion circuits, it has been found, as stated above, that the positioning of the alternating current axis of the picture signals is sometimes not absolutely accurate with the result that the proper direct currentcomponent is not in fact transmitted. This lack of absolutely correct insertion of r the direct current component exists by reason oi. the inherent finite impedance of the diode or peak rectifler that is normally used for theinsertion of the direct current component.

It is thereforeone purpose of the present invention to provide a circuit ior use at a television transmitter whereby the direct current component may be accurately reinserted in the picture signal series.

Still another purpose of the present invention resides in the provision of means whereby the position of the alternating current axis of the picture signals may be properly determined with respect to a reference point in accordance with the actual over-all brilliance of the particular television image scanned.

A still further purpose of the present invention resides in the provision of corrective networks whereby the inabllity oi. the usual direct current insertion diode to properly relate the alternating current axis may be compensated for to thereby introduce into the picture signal series the proper direct current level.

A still further purpose of the present invention resides in the provision of circuit means whereby both the alternating and the direct'current cornponents'oi a picture signalmay be properly and accurately transmitted both as to frequency and relative intensities. Still another'purpose of the present invention wesides in the provision of a corrective circuit for introducing a variable correction component, the effectiveness of the correction component being a function of the average light intensity oi the picture transmitted.

Various other purposes and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description particularly when considered in connection with the drawings wherein:

Figure 1 represents one form of the present invention;

Figures 2, 3 and 4 represent modified forms of the present invention; and,

Figure 5 shows curves used in explaining the operation of the present invention.

Referring now to the drawings and particularly to Figure 1, there is shown a television picture signal amplifying tube H) which includes a cathode li, a control electrode l4 and anode IS. The cathode is connected to ground by way of a biasing resistance It while the control electrode is also connected to ground by grid resistor 20. When picture signals are applied to the terminals 22 the signals are impressed between cathode i2 and control electrode H of tube Ill by way of coupling condenser 24. The picture signals modulate the electron stream in the amplifying tube It with the result that varying degrees of anode current are permitted to pass through the anode load resistance 26. The potential appearing at the anode it oftube ill is then impressed upon the control electrode 30 of tube 32 by way of coupling condenser 28.

The tube 32 preferably combines two triode elements including cathodes 34 and 38, control electrodes 30 and 38 and anodes 40 and 42. It"

is to be understood, or course, that two separate tubes may be used in place oi. the single twin" tube. -The cathodes 34 and 36 are connected together and are in turn connected to. ground sponds to the direct current component of the picture signal produced at the television transmitting tube and produces a control potential that is applied to the control electrode 30 or tube 32 so as to vary, ata relatively slow rate, the average potential of control electrode 30. The alternating current component of the picture signal series is impressed upon the control electrode 30 by way of condenser 28 and this component effects a rapid variation in the potential of the control electrode 30 of tube 32. 'In the circuit or the amplifier preceding the direct current inserter 48, only alternating current components are present and the picture signals vary between certain limits about a relatively fixed alternating current axis. Since, as stated above, coupling between amplifier stages is afforded by transformer or condenser means, the direct current component is lost and theaverage potential of 1 the picture signals remains relatively constant.

In the absence of the direct current inserter the trode II of tube 32 is affected by the relati slow potential variations and the direct current component to thereby shift the alternating current axis of the picture signal series. with the invention as so far, described there will then up relatively IOW'SO that the impedance of the diode pear across "the cathode potentiometer resistance 44 potential variations representative of'the alternating component and fairly representative of the direct current component. By reason of theimpedance ofthe diode in the direct current inserter'lfl the direct cln'rentlevel of the picture signals available from thepotentiometer It are t not absolutely correct.

, Examples of various forms of diode direct current inserting circuits are shown and described in U. S. patent to Willans et aL, No. 2,194,514,

issued March 26, 1940.

To aiford a complete and accurate understand- "ing of the invention, reference isnowmade to Figure 5 wherein there is'shcwn the grid voltageplate current characteristic curve (Er.-Ib) of an amplifier tube. Below this curve, and associated reinserter becomes appreciable. Whenthe picv ture is predominantly whitea larger amount of current is permitted ,to flowthrough the diode 1 reinserter with the result that the impedance of the diode is less predominant and a moreaccurate positioning of the alternating current axis results.

. For compensating for the undesired and inaccurate positioning oi the alternating current axis of the. picture signal component, reference is again made to Figure 1 and particularly to the triode section shown in the right hand half of tube. Since the cathodes 3C and 18' are connected together the'potential of the cathode 38 therewith, is shown a small portion of a picture signal series wherein the synchronizing signals are represented at S, the blanking or pedestal impulses or signals at B and the picture signals by the rapid potential variations between blanking impulses. Furthermore, the synchronizing signals extend in a negative direction whereas white in the picture extends in a positive dlrec-' tion. This potential relationship is the potential of the voltage variations present at the cathode. of tube 32 =tube.

and at the control electrode of the same It will be noticed by inspecting Figure '5 that the tops of the synchronizing impulses or signals correspond to a certain fixed potential represented by the dotted line 82. Regardless of the picture content the synchronizing signals are always transmitted at this fixed potential. Likewise the height of the blanking impulses. (that correspond to black in the picture) .are also transmitted at a relatively fixed potential represented by the vertical dotted line 84. These two potentials are substantially fixed and thealternating current axis of the picture signals is transmitted at a value related to one .or the other of these potentials by an amount determined by the average brilliance of the picture transmitted. In one instance, for example, picture signals such as shown at 68 may have an alternating current axis represented by the broken line 88 whereas picture signals such, as those represented at Ill may have an alternating current axis represented by the broken line 12. (For purposes of explanation, a large variation is shown in the background picture signals. When the usual diode direct current reinserter l8 alone is used it has been found that the position of the alternating current axes 88 and 12 are not accurately positioned but are, in fact, located generally a slight amount in the direction of black from their proper position. The inconsistency and variation in the inaccuracy in the positioning of the alternating current axis by the usual diode direct current re nserter 'mustvary exactly in accordance with the potential variations of cathode 34.

The control electrode38, however, is connected to a movable point 50 along potentiometer resistance 44 by a grid resistance 54. The control electrode 38 and the cathode 38 are also connected together by means of a filter condenser 56. The output from the tube 32 is derived from movable contact 52 associated with the potentiometer resistance element 44 in order that video or picture signal output may be derived-from terminals 80. Since it is not desired that the electron stream flowing between cathode 38 and anode 42 be modulated at the rapid potential variations of the picture signals,'the condenser 58 is provided so that the potential difference between control electrode 38 and cathode 36 may be made to vary at a relatively slow rate in accordance with the direct current component'of the picture. Any appropriate low. pass filter could as well be used. The anodic current flowing through this section of tube 32. however, is superimposed upon the anodic current flowing to anodelil inasmuch as a common cathode resistance 44 is used for each triode section of the tube. The current flowing to the anode .12 therefore modifies the potential drop across the cathode resistance 44 produced by the anode current flowing to anode 40.

It will now be assumed that a picture or image signal series including synchronizing andblanking impulses is impressed upon the control electrode 30 with the synchronizing signals extending in a negative direction and with white in the I picture extending in a positive direction. It will further be assumed that the picture or image signals represent an image that is predominantly black. Under these conditions the direct current inserter 48 will alter the potential of the control 48 is by reason of the fact that under conditions electrode 30 to shift the alternating current axis of the picture signal series but, as stated above. the shifted position may beslightly displaced from the position it should occupy. Since the i applied picture or image signals represent an image that is predominantly black, the average potential of the control electrode 3ll-will be somewhat negative (near the negative potential represented by the blanking signals). As a result, the average current that is permitted to flow from cathode 34 to anode 40 'will be relatively low so that a relatively low potential drop wil be produced across the cathode resistance 44. Since the bias of the control electrode 38 is a function o the average current in the cathode rcsstance H, and since under condit ons of signals representing a predominantly black picture the current in resistance 44 is low, then the bias of conof low background level such as represented by the picture signals shown at 88,.the amount of current permitted to flow through the diode is trol electrode as with respect to its cathodes a,

will be relatively low. As a result, an appreciablei amount of current will be permitted to flow. vfrom.

the cathodezlito-the anode fl. increased amount, of current when superimposed upon the current; flowing through 'cathod'e're'sis tance 4d and; anode .40- produces a further potential-"drop so-that the alternating current axis of 'thespicture I signals available at the terminal 60 will be shifted slightly in a direction oi'white 01' away irom'the potential representing full blackin 'the' picture; 'If, however, a picture signal series isapplied to turewith the result that moreaver'age -current is permitted to flow to the anode 4U. 'A '-larger-- potential drop will then be produced across oath-5 ode resistance dxincreasing the bias on the-cons trol electrode '33 with respect to its cathode. "TI-11871 increased bias is not sufiicient to causecut-cff-of the tube and anode current continues to flow to however, under some circumstances and under I0 is operated near'thecathode I! it may be n w: --toapply a .positive'potential to-the' controllel ctr-ode I! with respect toground; The circuit'sliown'in= Figure 2'operates' in a manner substantially idem U ticai to the'operationzot mum, 1, the average thecontrol electrode that correspondsto a trol electrode 338' isfcnnected byreanode 42. This current, when superimposed'upon' the current flowing through resistance; it etc anode causes a shift in the alternating current axis of the signals as applied to the control electrode 3D. This shift is in'the same direction its.v in the abov example where,signalsrepresentative of a predominantly blackpicture were applied to control electrode 30. The additional shift Furthermore, a resistance u 'is connec; as

operation of the circuit shown in Figure 3- it in the alternating current axis as a result of the use of the direct current amplifier is greaterflfor '1 signals representlng'a predominantly white picture than for a predominantly black"picture.

' .The elements 36, and 42 of tube and the manner in which they are connected in the cire cult constitute the elements of, and naturally junction as, an amplifier for the direct current and low frequency components;

in orderlthat the as 'oiffresistauce as to" y 7 v tentiometer resistance M may fbe manually cone potential'oi' the cathode 3i beinghontrolle'd w the position or the movablecontact "in order? to introduce varying degrees ot compensating" current into 'portionpf'the potentlmneter r sistance'b'etween" movable "contact I8 hud-the grounded end'oithe potentiometer. By'making the potential-oi the biasing source's: adjustable,

and-by means" or the movable contact varyingf degrees of compensating'eiiects may result;

A'still further modification is shown in Figure 3 andinthisil'gure asin Figure 2 elements-which? I are identical to the elements shqwninfll lgurefl bear thesame reference numerals; InthismOdi a sess; the cathodes 35 and *3. are connecteq f 'gethrbymeangjo! a. resistance Fie d the cone:

slstance 54. Ii condenser "'50, connected be tween control el ctrod :10 nd cathode name-r 1 to p h c ntrh l ct c 'i oi' n d t tweenthe' anodes" 5p an 4': more: or any possibility orpgsrssme serous;- For he preferable to make the resistance Iladiustabei trolled. The resistance 44 carries the). cathode current to both of thecathodes l4 and 36 where as resistance 35 is-traversed only by current to cathode 36. By properly proportioning the values of cathode resistance 44 and I8, therefore it is' possible to vary the efiects and the. amount of correction produced by the correction triode." In

J general,'the circuitshown in this modification rent component as produced by the television 4 transmitting tube. The operation of the 'triode the grounded end of resistance 84. The eiIect oi the correction triode is reduced to a minimum when the movable contact is positionednear the cathode end of the resistance 44,.

A modified form of the invention is shown in Figure 2 and elements corresponding to the eleput in the picture signal series is'accornplishedin' operates substantially the same as the circuit shown in Figure 1 except that the manner in which the effectiveness of the tube is controlled is somewhat diflerent. A still further modiflcation'of the present invention lssho wn' in Figure 4; wherein the correction oi the direct current-insertion. together with,

the provis'ion for aflording a low impedance out-f one and the'samefelectron stream. The tube 14 includes a cathod jt'lt, a first control electrode'll; a second control electrode II and an anode 82. The anode is 'connected'to a terminal' to which a' positivepmntial is app ed whereas'thei cathode -18, is'connected to ground by way'o! potent'iometer. resistance 84. The control electrode 80 is connected'tomovable contact 50 by way of resistance 54 while the same control electrode is connectedto the cathode "IIhy Way of con-L denser E3. The tube alsoincludes 'screenand suppressorelectrodes, the screen'electrodes being ments oi Figure 1 bear identical reference numerals. This. circuit di'flfers from the circuitv shown in Figure 1 in that the cathodes 34 and l4 and the control electrode 38 is connected to ground by way of resistance" and biasing potential 55. Normally, the biasing potential or when picturesignalg are decoupled by means of resistance. E5 arid decoupling condenser 88. The outlmt rromthe'cathodel resistance M is derived from the movable contact 52 through terminals to. 1

wished to'the control electrodes II and substantially correct direct 0112'."

rent reinsertion is applied, a voltage drop is pro-J duced across cathode resistance. A certain p rtion of this voltage-drop is; then applied to the 7 control electrode 89 to vary the average potential the source 55 may besuch astomaintain the conv trol'electrode 38 negative with respect to ground, 1-6 Thisslow variation in'thepotential or conf oil-the control electrode ll with respect to cathode 3 aseaaoo trol electrode as with. respect to'its associated cathode thereby modifies the average amount of current that is permitted to pass through thetube with the result that the potential drop across re-' sistance I4 is altered in.-accondance with the'direct current. component of thepicture signal as isthecaseinthecircuitshowninFlgure 1. The

circuit shown in Figure 4 is somewhat similar to the circuit shown in Figure 1 except that a single electron stream is used and two control electrodes are provided for eflecting modulation of'the intensity of the electron'stream.

The choice of the size of the resistance 54 and condenser "in each of the modificationsis preferably such as to provide a time constant circuit having a time constant of the order of 0.1 to 1.0

used.

, second. The rate at which the control electrode 7 associated with this time constant circuit is permittedto change potential is therefore relatively sented by black in the picture in accordance with the true light value or background level of the I television picture being transmitted. The circuits are used for compensating for inaccuracies 'introduced by the usual diode direct current reinserting device since theoretically it is impossible to obtain completely accurate reinsertion of the direct current component by a diode reinserting device alone.v Certain refinements have been made in order to aiiord a more accurate insertion'of the D. C. component and such circuits are, for example, shown in the United States patent of F. M. Deerhake, No. 2,302,425, issued November 1'1, 1942. Normally, however, the load circuit .for the diode reinsertion tube is of the .order of one megohm so that an appreciable error may be introduced in the insertion of the direct current level.

I, If, however, the load impedance is increased to the order of 10 megohms the percentage of error is reduced,'however, it is not completely eliminated. It is, however, sometimes disadvantageous to use high impedance load circuits so that with the present invention it is possible to use a load impedance of the order of one megohm and still assure proper reinsertion of the direct current component irrespective of the background level of the picture under consideration.

The apparatus represented schematically by the reference numeral 48 has been described and referred to as a diode direct current reinserter. It is possible, however, that this element may take urally follow that the signals could bederived from the anodes 40 and of tube 32 or from the anode 82 of tube I4. Under these circumstancesan anode load impedance would, of course, be Although the present invention is described in Jmore or less detail, it is to be understood that various alterations and modifications of the pres-' ent invention may become apparent to those. skilled in the art and it is desirable that any andv all such modifications and alterations be considered within the purview of the present inven pended claims.

WhatIclaimis: r 1. A television transmitting apparatus wherein tion except as limited by the hereinafter, ap-

a television image signal series is generated which V a spaced intervals assume a value equal to the threshold value of the blacker than black? region,

ting direc'tcurrent for amplifying the alternating prising an amplifier for amplifying the alternat-' comprising an amplifier incapable of transmitcurrent components of said image signals, means at the output sideof saidamplifier responsive to the peak values of said blacker than black interval signals for reinserting a fraction of the "di rect current component into said image signal series, and a direct current amplifier responsive to the fractionally reinserted direct current component for completing the -reinsertion of the direct current component.

2. A television transmitting apparatus wherein a composite series of signals including television image signals interspersed with synchronizing impulses are supplied, the intensity of the synchronizing impulses being substantially uniform, coming current component of the composite series of signals, said amplifier being incapableof transthe form of the so called clamp direct current reinserter.. An example of a clamp circuit for reinserting the direct current component, as well as a clear and concise description of such an apparatus is to be foundin Collard U. S. Patent No. 2,284,085, issued on May 26, 1942. The present invention may be used to assure proper reinsertion of the direct current component where the initial reinsertion is performed by a clamp circuit in place of a diode or peak rectifier.

In all of the figures showing the various forms of the present invention the output circuit is shown connected to the cathode resistance. Such a connectionwould supply signals for applicamitting the'direct current'and low frequencyalternating current components, means at the out-. put of the amplifier for initially positioning the alternating current axis of the amplified signals with respect'to black by partial reinsertion of the direct current and low frequency alternating current components, and a direct current amplifier responsive to the partially reinserted direct current and low frequency alternating current components. for accurately positioning the with synchronizing impulses are supplied, the in- ,tensity of the synchronizing impulses being subcurrent amplifier responsive to the partially re-- inserted direct. current component for accurately positioning the zero axis of the alternating current component with respect to black.

4. A television transmitting apparatus wherein a series of television image signals interspersed with synchronizing signals are generated, the intensity of the synchronizing signals being substantially uniform, comprising an amplifier for amplifying the alternating current component of however, it is desired to supply signals tea high impedance translating device then it wouldnot 7 current components.

ing current components, and a the image signals an synchronizing signals, said amplifier being incapable of transmitting the.,

1 direct current component, means at the output side oi jtheampliiler ior initially shitting the zero axis otthe alternating current component with respect to a .reierencepotential by partial a direct current ampliiier responsive to the ini-" tially ,sliiited alterna in current axis for completing the reinsertionoi the,. direct current com- 'reinser'tion ofthe direct current component,; and

' e-,ses,soo

coupled electron emissive means responsive the reinserted portion or the direct current component for completing the reinsertion of the direct. current and lo w frequency alternating. cur-z,

rent components.

i n television transmitteryiherein seriesof television im'agesignals and synclii' oniz- {H in; impulses interspersed therewith are-supplied andgwherelthe polarity oithesynchronizing impulses i O posite to that of the image signals, comprising means for amplifying the ,alternatponent to thereby accurately position the zero axis oi the, alternating current component with respect to the referencepotential; p 5. A television transmitter whereina composite I series of television imagesignals antl syrichroniz v and'where themolarity ofthe synchronizing impulses is opp ositeto that of the image signals,

ing impulses interspersed therewith-areisupplied current component of the image signals and syn; chronizing impulses, said ampliiyingmeansbelow frequency alternating, current wcomponentsi means responsive to thepeaks of impulse maxima in one direction to supply aportionioithe missing ing. current component of the image signals and. n ron z puls s a p i yinstmes a being incapable of. transmitting the direct g cur-,1- rent and low frequency alternating. current components oL the composite series, 13188118418813011 sive to the pealrs of impulse maxima'iina predecomprising means for amplifying the alternating ing'incapable of transmitting directcurrentand termined direction to v reinsert a portion of the direct current and low. irequencynlternating rentcomponents into the composite series, and means responsive to mesmerized portion of the direct ,currentucomponent for completing there-g insertion ofthedirectcurrent and low ireguency direct current and, low irequency-altemating cur.-

rent components, and a direct current 'ampIifler responsive to the reinserted direct current component portion for completing the reinsertion oi comprising means for amplifying the alternatl ing-current component of the image signaisand' synchronizing impulses, said amplifying means beingincapable of transmitting the direct current and low frequency alternating current components of the composite series, means responsive to the peaks of impulse maxima of a predetermined polarity to reinsert a portion or the miss ing direct current and low frequency alternat direct current alternating current components; into the -comp slte's a. {a television transmitt r. w rein television image signals and synchronizing impulses inter-,;

spersed therewithare generated .and wherethe':

polarity of the-synchronizin mpulses in 099- site to that of the image signals, comprising means ior amplifying the alternating current 'ing impulses, said amplifying means beingincomponent of theiinag'e signals andsynchronizcapable of transmitting, direct current and low frequency alternating current signal components, means responsive to the peaks of impulses oi the polarity. of the synchronizing impulses to rein-.

sert aportion 01' the missing direct current and low frequency alternating current signal components, anda'direct current coupled electron emissive device responsive to the reinserted direct current component portion for completing the reinsertion of the direct current and low ire quency alternating current components. 4s 1 LEv RQY MOFFETT. 

